Cirrostratus clous are thin, generally uniform clouds, composed of ice-crystals, capable of forming halos. When thick enough to be seen, they are whitish, usually with no distinguishing features. When covering the whole sky and sometimes so thin as to be hardly discernible, this may indicate a large amount of moisture in the upper atmosphere. Cirrostratus clouds sometimes signal the beginning of a warm front and thus may be signs that precipitation might follow in the next 12-24 hours. These clouds are located above 6,000m (20,000ft). Compare cirrostratus with other stratus cloud formations at lower altitude: altostratus, nimbostratus, and low altitude stratus clouds.